It has been known that when a denture is used for extended periods of time, the shape of the oral cavity gradually varies due to dissolution of alveolar bone that is forming alveolar ridge into the oral tissue. In such a case, fitness becomes poor between the denture base and the oral mucosa, and the denture loses stability. If the poorly fit denture is continuously used, non-uniform pressure is exerted on the mucosa that comes in contact with the denture. Therefore, an ulcer or an inflammation occurs in the mucosa, and a pain is triggered by the force of occlusion. Therefore, in case such an unfitness has occurred, it becomes necessary to recover the fitness of the denture to the mucosa by preparing a new denture or by relining the denture that is in use.
However, the oral mucosa of a patient who is suffering from a serious ulcer or inflammation is in a very unstable state. Therefore, a favorable fitness must be maintained between the denture base and the mucosa prior to preparing a new denture or relining the denture until the oral mucosa (tissue) recovers to a relatively healthy condition. The tissue conditioner for dental use is used for such a case. Namely, the tissue conditioner for dental use is a therapeutic material used for relining the denture that is now in use until the form and color tone of the mucosa under the denture base are recovered to a normal state.
Soft materials now used in connection with the denture bases can be roughly divided into a paste for fixing the denture base (so-called denture adhesive), a tissue conditioner for dental use and a soft relining material, which are used for different purposes, for different methods, for different periods of time and for providing different properties.
The paste for fixing the denture base is applied by the patient himself for relieving the pain, and is used for only a very short period of time. Use of this material is limited to be only temporarily and is not for the purpose of therapy. The material can be used in a powdery form, in a cream form, in a seal form, and is either water-soluble or water-insoluble. The paste increases the viscosity of saliva and works to increase the adhering force of the denture base to the surface of the mucosa. The water-insoluble paste eliminates the gap between the denture base and the surface of the mucosa to improve fitness, and increases the adsorbing force based on the effect of marginal adaptability. These pastes have almost no elasticity, and undergo a large plastic deformation for each occlusion to a degree that depends upon the pressure of occlusion. Therefore, their use is limited to one day to several days.
On the other hand, the tissue conditioner for dental use and the soft relining material have a common point in that they are applied by a dentist but exhibit properties that are greatly different depending upon the objects of use.
Concretely, the tissue conditioner for dental use (hereinafter often simply called tissue conditioner) is used for the therapy of the oral mucosa in a stage that precedes the repair of denture components as described above. The tissue conditioner is a soft high-molecular material which is applied to the surface of the denture base that comes in contact with the mucosa to release the distortion and indentation of mucosa under the denture base and that varies its state following the displacement of the mucosa. The tissue conditioner is used in the form of a paste formed by mulling a powder and a liquid material together. The paste exhibits a high fluidity at first. As the time elapses, however, the liquid material infiltrates into the powder and viscoelasticity is exhibited. Therefore, the paste is applied onto the surface of the denture base while it still has fluidity, and is inserted in the oral cavity to impart the shape thereto. The period of use is from about several days to about several weeks until the oral mucosa recovers to a healthy condition. The tissue conditioner must not be pushed out from between the denture and the surface of the mucosa at the time of occlusion but must remain flexible and capable of undergoing fine deformation so as to follow the recovery of the oral mucosa yet being held on the surface of the mucosa. If described in further detail, the tissue conditioner is relined onto the surface of the denture base that fits poorly in order to recover the fitness between the denture and the oral mucosa to wait for the gradual diminishing of ulcer and inflammation of the oral mucosa while relieving the pain. As the ulcer and inflammation of oral mucosa gradually diminish, the state of the oral mucosa recovers with the passage of time to resume the original state. Here, the tissue conditioner must undergo the plastic deformation to meet a change in the state of the oral mucosa. This is because, if the tissue conditioner does not undergo the deformation that is described above, the fitness is lost as the state of the oral mucosa recovers creating a factor that causes a pain again.
On the other hand, the soft relining material is a material that is used when the oral mucosa is in a healthy condition. When the ulcer or inflammation is occurring in the oral mucosa, the tissue conditioner is used until the ulcer or inflammation extinguishes, i.e., to wait for the extinction of the ulcer or inflammation. After the ulcer and inflammation have extinguished and the oral mucosa has resumed the healthy condition, the soft relining material is used. Therefore, when neither the ulcer nor the inflammation is occurring, the soft relining material is readily used without using the tissue conditioner. That is, the tissue conditioner is applied to the denture base as a temporary relining material for a period of several days to several weeks until the ulcer or the inflammation extinguishes, whereas the soft relining material is used as a relining material for repairing the denture base when the oral mucosa is in a healthy condition to exhibit its function for at least six months. Therefore, the characteristics of the two are obviously different.
For example, the tissue conditioner is used when the ulcer or the inflammation is occurring, and is very soft. From the standpoint of suitably holding the denture for extended periods of time or from the standpoint of the feeling of use of the denture, however, the tissue conditioner is too soft. Further, the oral mucosa after recovered does not so change as during the stage of from the occurrence of ulcer until the recovery thereof. Therefore, a high degree of plasticity possessed by the tissue conditioner adversely affects the holding of the denture or the feeling of use. Therefore, the soft relining material used for the repaired denture base is a soft material which, however, is slightly harder than the tissue conditioner and does not exhibit such a large plastic deformation as that of the tissue conditioner. Besides, the soft relining material is used for a period of time longer than the tissue conditioner and, therefore, has mechanical properties such as tensile strength and the like which are very higher than those of the tissue conditioner.
As described above, the above three kinds of materials are all soft materials and are used for relining the denture, but are used as clearly different dental materials from the standpoint of their purposes of use and required properties.
As described above, though so high strength is not required, a high degree of softness and plasticity are required for the tissue conditioner. The tissue conditioners of a variety of compositions have been proposed for satisfying the above properties. From the standpoint of simplicity at the time of use, however, many of the tissue conditioners now used are those comprising a (meth)acrylic polymer powder and a liquid material of various plasticizers as chief components (see, for example, non-patent document 1, and patent documents 1 and 2).
The plasticizers are the components necessary for exhibiting softness and plasticity. At present, a phthalate-type plasticizer has been chiefly used. For example, a paste of a powder component comprising a (meth)acrylic polymer powder such as a polyethyl methacrylate or a copolymer thereof and a liquid component comprising a phthalate-type plasticizer containing ethanol in an amount of about 4 to about 30 mass %, has been widely used as a tissue conditioner. The liquid component is, in many cases, blended with ethanol to improve kneading property and property after kneaded.
However, the tissue conditioner using the above plasticizer has a problem in that the plasticizer gradually elutes out (e.g., see non-patent document 1).                Non-patent document 1: “Factors Affecting the Leaching of Phthalic Ester of the Experimental Tissue Conditioner” (N. Kawaguchi and three others), Dental Material and Device, Japanese Society of Dental Materials and Devices, July 2004, Vol. 23, No. 4, pp. 273-278.        Patent document 1: JP-A-3-20204        Patent document 2: JP-A-2-297358        
That is, if the plasticizer elutes out, the paste of the tissue conditioner loses viscoelasticity with the passage of time and is cured. The elution occurs in a relatively short period of time of about several days and becomes a serious problem for the tissue conditioner which is used for a period of time shorter than the soft relining material and for which more softness is required than the soft relining material. Therefore, the tissue conditioner had to be renewed every after several days. It has further been suspected that the phthalic acid ester-type plasticizer exhibits estrogen-like action. The plasticizer eluted out from the tissue conditioner infiltrates into the body and may affect the health. Moreover, if the material containing the plasticizer that is left to stand in contact with any other substance, the plasticizer may diffuse and may transform into other substances. Therefore, if the tissue conditioner containing the plasticizer is used in the oral cavity being stuck to the denture base, the plasticizer gradually migrates into the denture base. As a result of migration of the plasticizer, the denture base is softened and is deteriorated.
As a plasticizer to substitute for the phthalate-type ones that may affect health, a sebacate-type plasticizer has recently been used without, however, solving the problems of a decrease in the softness caused by the elution of the plasticizer or the deterioration of the denture base caused by the migration of plasticizer into the denture base.